1. Field of the Invention
This invention relates to improvements in interferometer gyroscopes, and, more particularly, to improvements in interferometer gyroscopes of the type in which a phase difference between counter-rotating light beams modulated with known frequencies is developed to determine the rotation of the optical path through which the beams traverse.
2. Description of the Prior Art
In the advance of interferometer gyroscope, or gyro, art, interferometer gyros have been proposed using a light path formed of a ring of fiber optic material through which counter-rotating light components travel. One such gyro is described in U.S. patent application Ser. No. 936,680 filed Aug. 23, 1978, assigned to the assignee of the present invention and incorporated herein by reference. In that application, a light beam from a light source, such as a source of laser or coherent light, is divided, and signals of first and second frequencies are impressed or modulated onto respective ones of the divided components. The modulated signals are introduced into the fiber ring in opposite or counter-rotating directions, then detected after each has traversed the length of the ring. The phase difference between the two detected components is then measured as an indication of the rotation of the fiber ring. The principle upon which the operation of the gyroscope is based is as described in U.S. Pat. No. 4,013,365; namely, that as the optical path is rotated and its apparent length is increased as seen by the light traveling in the direction of rotation, and is decreased as seen by the light traveling in the opposite direction. The increased and decreased apparent optical path lengths result in a phase change of equal but opposite amount upon the light in traversing the optical fiber ring.
One of the problems which has been found in the gyros of the prior art, especially for application in inertial grade apparatuses, is that it is difficult to achieve a sufficient dynamic phase detection range with linearity over the wide range of rotation rates anticipated; for example, rotation rates may range from 3.times.10.sup.-6 degrees per second to 300 degrees per second for a typical inertial navigation system sensor.
One laser gyroscope for measurement of inertial rotation is described by S. Ezekiel and S. R. Balsamo in an artical entitled "Passive Ring Resonator Laser Gyroscope" in applied Physics Letters, Volume 30, No. 9, page 478 on May 1, 1977. Ezekiel and Balsamo describe a system which employs two independently controlled laser frequencies to measure the clockwise and counterclockwise resonant frequencies of the passive ring. The light from a laser light source is divided and modulated by acousto-optic crystals in each path to produce a first light beam at frequency f.sub.0 +f.sub.1 and a second light beam of frequency f.sub.0 +f.sub.2. The frequency f.sub.2 is controlled through a feedback path to maintain the maximum output light detector. The frequency f.sub.2, however, is adjusted outside the loop which forms the passive ring. This, therefore, results in a system in which changes affecting one of the counter rotating light beams may not affect the other and, therefore, may result in measurement error.